[PATCH v2 5/5] Linux: Consolidate auxiliary vector parsing

[H.J. Lu]

On Fri, Feb 11, 2022 at 5:49 AM Adhemerval Zanella via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
>
>
> On 11/02/2022 10:24, Szabolcs Nagy wrote:
> > The 02/11/2022 09:47, Adhemerval Zanella wrote:
> >> On 11/02/2022 09:31, Szabolcs Nagy via Libc-alpha wrote:
> >>> The 02/03/2022 12:08, Florian Weimer via Libc-alpha wrote
> >>>> +  dl_parse_auxv_t auxv_values = { 0, };
> >>>> +  _dl_parse_auxv (av, auxv_values);
> >>>
> >>> this seems to use memset now on aarch64 before irelative
> >>> relocs are resolved in static binaries.
> >>>
> >>> which causes infinite loops in the iplt (i've also seen
> >>> segfaults in the build log).
> >>>
> >>> i wonder what is a clean fix...
> >>>
> >>
> >> Maybe add inhibit_loop_to_libcall to avoid the memset call.
> >
> > does not work for me..
> > happens at -O0 too
> > https://godbolt.org/z/W9r3nffYd
>
> Yeah, this won't help much this situation.  Unless we can make gcc *not*
> emit this memset call, we will need something like:
>
> diff --git a/elf/dl-support.c b/elf/dl-support.c
> index 1977a2be76..49f3fd2a61 100644
> --- a/elf/dl-support.c
> +++ b/elf/dl-support.c
> @@ -19,6 +19,10 @@
>  /* This file defines some things that for the dynamic linker are defined in
>     rtld.c and dl-sysdep.c in ways appropriate to bootstrap dynamic linking.  */
>
> +#ifndef SHARED
> +asm ("memset = __memset_generic");
> +#endif
> +
>  #include <string.h>
>  /* Mark symbols hidden in static PIE for early self relocation to work.
>     Note: string.h may have ifuncs which cannot be hidden on i686.  */
>
>
> $ make -j24 && make test t=elf/tst-tls1-static-non-pie
> [...]
> PASS: elf/tst-tls1-static-non-pie
> original exit status 0
> set bar to 1 (LE)
> get sum of foo and bar (IE) = 1
> get sum of foo and bar (LD or TLSDESC) = 1
> get sum of foo and bar (GD or TLSDESC) = 1
>
>
> I think the best approach would to add a __memset_generic, __memcpy_generic,
> and __memmove_generic on all targets and add a header similar to
> sysdeps/generic/symbol-hacks.h to activate the redirection when required
> (to not pessimize code that would work with the IPTL generated by the
> iFUNC).

I ran into a similar problem on my CET branch.  In my case, it is memcmp.
I added a new header file, <dl-start.h>, to define functions which can be safely
used during ld.so startup:

https://gitlab.com/x86-glibc/glibc/-/commit/2bfa258b114e556ab0be68d6465b5e4fa5504e2e


--
H.J.